Patent GB-376791-A (GB)
Title: Simple or double helical toothed wheel gear
IPC: F16H55/08
Filed: 1931-01-12
Abstract: 376,791. Toothed gearing. ROANO, F., 23, Via Giuseppe Silvati, Naples, Italy. Jan. 12, 1931, No. 1068. [Class 80 (i).] In single or double helical toothed gearing wherein the velocity ratio is equal to, greater or less than the inverted ratio of the diameters, the following contacts are separately obtained : (1) Along the whole of one or both of the arcs of intersection a c T, T z r of the wheels at or near their peripheries ; (2) along the arc a c and an inclined line c z situated at a predetermined fixed distance from the line of centres ; (3) along the arc a c, line c z, and arc z r ; (4) along the line c z and the arc z r. The same contacts are obtained to the right of the line of centres. The method of obtaining the contacts is shown and described with reference to a pair of gears in which the velocity ratio is 1 : 18 and the ratio of pitch circle diameters is 8À06:1, this inequality being obtained by providing a greater helix angle on the driving than on the driven wheel, in accordance with Specification 321,672, [Class 80 (i), Gearing, Belt &c.]. To obtain first the contact arc T c a, the plan of the helices is drawn in Fig. 4. The inclination of the teeth of the driving wheel varies from 24‹ at the point to 31‹ at the root, and the inclinations y, y, y<11> &c. corresponding to selected lines 1 .. 4 projected from Fig. 1, are calculated and drawn in broken lines through the pitch point 8, in Fig. 4. Contact along the arc T c a corresponds to contact along the arbitrarily chosen point A<1> A<1> of the teeth of the driven wheel for which the requisite thickness of the driving wheel teeth at the appropriate depth is 1, 5 ; 2, 6 ; 3, 7 &c., Fig. 4, which thickness is marked off on each side of a medial line b b, Fig. 5, along the projections 11 .. 41 of the selected lines 1 .. 4, Fig. 1. This gives the shape of a driving tooth, Fig. 5, seen in the direction of the arrow F, Fig. 1. To add also contact along the arc T z r and complete the contact (1) above, the same process is repeated using the points F F<1> of the driving wheel as datum to obtain the shape, Fig. 8, of a gap between the driven wheel teeth. To obtain contact (2), arc a c and line c z, the width of the gaps of the driven wheel teeth is increased to M M, M<1> M<1>, Fig. 4, corresponding to Q V, Fig. 12. Through Q and V lines Q S, V Z are drawn tangential to the gap sides G H L obtained in Fig. 8. Mating of this trapezoidal tooth gap with the tooth of Fig. 5 gives contact only at the point H, the projection of which on Fig. 1 is the point z. To restore contact over the arc T c a the driving teeth, Fig. 5, are uniformly thickened corresponding to the new thickness 5g, 6, &c., Fig. 4, giving the curved profile shown in Fig. 13. The thickness of the points of the teeth is again reduced to F<1> F<11> corresponding to F F<1>, Fig. 5, and from these points straight lines are drawn tangential to the newly thickened profile, Fig. 13. Trapezoidal driving teeth F<1> F" R P, Fig. 13, are obtained, and these mating with the trapezoidal gaps V Z S Q, Fig. 12, give contact also at the point D, the projection of which on Fig. 1 is c. Since the teeth are straight sided and contact has already been established at z contact occurs over the line c z. To add also the arc a c and so obtain contact (2), above, the trapezoidal driving tooth, Fig. 13, is modified by adding to its root at each side the material R C D. To add to this also the contact arc z r and obtain contact (3), above, the material H L S is added to the bottom of the trapezoidal driven tooth gap, Fig. 12, at each side. To obtain contact (4), above, line c z and are z r, trapezoidal driving teeth F<1> F<11> R P, Fig. 13, mate with gaps, Fig. 12, modified as for (3) above. The same construction mav be used for normal helical gearing in which the velocity ratio is equal to the inverted ratio of the diameters since the helix angle still varies with the depth of the tooth. The invention may be applied to internally toothed wheels. The points of the teeth may be rounded, blunted or faceted to increase contact.